# 3.9. Query Servers

## 3.9.1. Query Servers Definition

`[query_servers]`

  Changed in version 1.2:: Added CoffeeScript query server

  CouchDB delegates computation of design documents functions to external query servers. The external query server is a special OS process which communicates with CouchDB over standard input/output using a very simple line-based protocol with JSON messages.

  The external query server may be defined in configuration file following next pattern:

      [query_servers]
      LANGUAGE = PATH ARGS

  Where:

  - `LANGUAGE`: is a programming language which code this query server may execute. For instance, there are python, ruby, clojure and other query servers in wild. This value is also used for ddoc field `language` to determine which query server processes the functions.

    Note, that you may set up multiple query servers for the same programming language, but you have to name them different (like python-dev etc.).

  - `PATH`: is a system path to the executable binary program that runs the query server.

  - `ARGS`: optionally, you may specify additional command line arguments for the executable `PATH`.

  The default query server is written in JavaScript, running via Mozilla SpiderMonkey:

      [query_servers]
      javascript = /usr/bin/couchjs /usr/share/couchdb/server/main.js
      coffeescript = /usr/bin/couchjs /usr/share/couchdb/server/main-coffee.js

  See also

  Native Erlang Query Server that allows to process Erlang ddocs and runs within CouchDB bypassing stdio communication and JSON serialization/deserialization round trip overhead.

## 3.9.2. Query Servers Configuration

`[query_server_config]`
  `commit_freq`

    Specifies the delay in seconds before view index changes are committed to disk. The default value is `5`:

        [query_server_config]
        commit_freq = 5

  `os_process_limit`
  `limit`

    Hard limit on the number of OS processes usable by Query Servers. The default value is `100`:

        [query_server_config]
        os_process_limit = 100

    Setting os_process_limit too low can result in starvation of Query Servers, and manifest in os_process_timeout errors, while setting it too high can potentially use too many system resources. Production settings are typically 10-20 times the default value.

  `os_process_soft_limit`
  `soft limit`

    Soft limit on the number of OS processes usable by Query Servers. The default value is `100`:

        [query_server_config]
        os_process_soft_limit = 100

    Idle OS processes are closed until the total reaches the soft limit.

    For example, if the hard limit is 200 and the soft limit is 100, the total number of OS processes will never exceed 200, and CouchDB will close all idle OS processes until it reaches 100, at which point it will leave the rest intact, even if some are idle.

  `reduce_limit`

    Controls Reduce overflow error that raises when output of reduce functions is too big:

        [query_server_config]
        reduce_limit = true

    Normally, you don’t have to disable (by setting `false` value) this option since main propose of reduce functions is to reduce the input.

## 3.9.3. Native Erlang Query Server

`[native_query_servers]`

  Warning

  Due to security restrictions, the Erlang query server is disabled by default.

  Unlike the JavaScript query server, the Erlang one does not runs in a sandbox mode. This means that Erlang code has full access to your OS, filesystem and network, which may lead to security issues. While Erlang functions are faster than JavaScript ones, you need to be careful about running them, especially if they were written by someone else.

  CouchDB has a native Erlang query server, allowing you to write your map/reduce functions in Erlang.

  First, you’ll need to edit your local.ini to include a `[native_query_servers]` section:

      [native_query_servers]
      erlang = {couch_native_process, start_link, []}

  To see these changes you will also need to restart the server.

  Let’s try an example of map/reduce functions which count the total documents at each number of revisions (there are x many documents at version “1”, and y documents at “2”… etc). Add a few documents to the database, then enter the following functions as a view:

      %% Map Function
      fun({Doc}) ->
          <<K,_/binary>> = proplists:get_value(<<"_rev">>, Doc, null),
          V = proplists:get_value(<<"_id">>, Doc, null),
          Emit(<<K>>, V)
      end.

      %% Reduce Function
      fun(Keys, Values, ReReduce) -> length(Values) end.

  If all has gone well, after running the view you should see a list of the total number of documents at each revision number.